Replace the homegrown GIF parser and LZW decompressor with the wuffs
GIF decoder, which is memory-safe by construction and already used in
other engines via Skia.
One behavior change is that `loop_count()` now reports the correct
value, since the raw value stored in the file does not include the
first frane and should be incremented by 1 to be compatible with what
callers expect.
Move the image loader sources and decoder-only dependencies from LibGfx
into a new LibImageDecoders library. This keeps the APNG-enabled PNG
loader out of processes that only need core graphics and image writers.
Link the ImageDecoder service, direct decoder tests, fuzzers, test-web,
and the image utility against LibImageDecoders where they still decode
images in-process.
